The present invention relates generally to optical techniques for measuring the thickness of optical films, and more particularly, to apparatus and methods using nondestructive optical techniques for simultaneously measuring optical constants and thicknesses of single and multilayer films.
Currently, there are several methods to determine the optical constants of thin films. These methods are described in the following references. The "Handbook of Optical Constants of Solids", edited by E. D. Palik (academic Press, NY, 1985) describes measurement of normal-incidence reflectance and transmittance over a wide spectral range.
The "Handbook of Optical Constants of Solids II", edited by E. D. Palik (academic Press, NY, 1991) describes measurement of R and T for normal and oblique angles of incidence (45.degree.; 60.degree.) for the polarizations TE and Tm, over a wide spectral range. In "Physics of Thin Films", Vol. 2, by O. S. Heavens, edited by G. Hass and R. E. Thum (Academic Press, NY, 1964), using ellipsometry to measurement the polarization states of collimated monochromatic light before and after reflection from a surface to obtain the ratio r=R.sub.P /R.sub.S,=tan .psi. exp(i .DELTA.) of the complex p and s reflection coefficients is discussed. These methods of determining the optical constants (n and k), however, are complicated and at times yield inaccurate results.
The major disadvantage of the first two methods is that the optical constants are determined from the magnitude of the reflection spectrum which cannot be measured accurately (.DELTA.R.about..+-.0.3%). The measurement errors in the reflection magnitude introduce significant errors in the optical constants (.DELTA.n.about..+-.1%).
In general, the three above-cited methods are not appropriate for determining simultaneously and accurately the optical constants and thickness of relatively thick layers (greater than 20.lambda.; greater than 10 .mu.m, where .lambda. is the wavelength).
It would therefore be desirable to have an optical film thickness measuring apparatus and method that overcomes the limitations of conventional approaches. Accordingly, it is an objective of the present invention to provide for nondestructive optical techniques, including apparatus and methods, for simultaneously measuring optical constants and thicknesses of single and multilayer films.